The purpose of this study is to determine whether thymic transplantation in addition to highly active antiretroviral therapy (HAART) will lead to restoration of T cell function in HIV infection. Initially HIV-infected patients were enrolled who had CD4+ T cell counts between 200 and 500/cumm. After 3 weeks of HAART, the patients were randomized into two groups: 4 patients received cultured allogeneic postnatal thymic tissue at day 42 of HAART and the other 4 were controls. Following 1 week of HAART, patients were immunized every 6 months with the neoantigen keyhole limpet hemocyanin (KLH) and the recall antigen tetanus toxoid (TT). Follow up studies included HIV plasma RNA, CD4+ T cell counts, and T cell proliferative responses to immunizations with recall and neoantigens. Biopsies were done of the transplanted thymus. Currently, only HIV-infected patients with CD4+ T cell counts less than 100 CD4+T cells are being enrolled and these patients are already on HAART and have HIV plasma RNA less than 400 copies/ml. These patients receive peritransplant immunosuppression to prevent graft rejection. With this study we hope to determine how to restore immune function in patients with low T cell counts despite HAART. HIV-infected patients in this category remain at risk for life threatening infections.